The production of nanocrystalline cellulose (NCC) from several cellulose sources including wood pulp involves an acid hydrolysis step. Depending on the starting cellulosic material, a considerable amount of acid such as sulphuric, nitric, phosphoric, or hydrochloric acid can be employed. Nanocrystalline cellulose has very interesting and unique properties different from those of pulp fibres and microcrystalline cellulose. It can be used in several applications.
Typically, a sulphuric acid concentration of between 50 and 70 wt % is employed. After the separation of the NCC particles, a solution rich in sulphuric acid and sugars is obtained. This spent acid stream is free of suspended solids and contains mainly sugar oligomers, sugar monomers and acid; this stream is typically considered a waste stream presenting disposal problems.
It would reduce the operating cost of the NCC process and the discharge of waste streams to the environment, if the acid could be separated and recycled to the NCC process. This would require that the acid be concentrated by evaporation to its original level, but if the sugars are not separated from the acid, concentrating the acid leads to the degradation of these sugars by dehydration leading to the formation of products like furfural and hydroxymethylfurfural and low molecular weight organic acids. In addition, fouling of the heat transfer area during the acid concentration step is possible due to sugar caramelisation during the evaporation process.
The fermentation of monomeric sugars in the presence of oligomers is not an easy process to perform since the latter act as fermentation inhibitors. Oligomers are polymeric carbohydrates having a degree of polymerisation of 2-10. The separation of the sugar monomers from the sugar oligomers is a desirable and an attractive option which may be of use in the production of other value-added products. Monomeric sugars can be fermented to produce ethanol. Oligomeric sugars can be used for instance in the food and pharmaceutical industries. Recently, a growing interest is being given to oligosaccharides due to their nutritional benefits when added as ingredients in some foods. Cellulose oligomers (e.g. cellobiose) are known to act as pre-biotics when added to animal feed. The incorporation of cellobiose in medical drugs, food, and cosmetics is being developed in Japan. Other studies showed that administrating fructo-oligosaccharides and galactooligosaccharides can increase the number of useful bacteria in the colon while suppressing the number of harmful bacteria.
Xylooligosaccharides can be utilised as prebiotics and tend to lower the risk of colon cancer. Xylooligosaccarides are used as food ingredients and tend to lower cholesterol levels. It was reported that human milk may contain at least twenty-one different kinds of oligosaccharides. These oligosaccharides play a vital role in infant growth and the development of the immunisation system. Several studies have been conducted and showed the benefits of adding galacto-oligosaccharides to cow's milk based infant formula. The incorporation of these sugar oligomers in dairy products and desserts is increasing worldwide due to the increase in consumer health consciousness. Oligosaccharides have also been incorporated in cosmetics for skin treatment (EP0591443).
The separation of sugars from acid solutions in different applications has been investigated in the prior art. Several approaches such as ion exchange have been devised. Most of the work in the previous art dealt with biomass hydrolysate solutions.
For example, U.S. Pat. No. 5,580,389 discusses the separation of acid from sugars from strong acid hydrolysis of biomass. The method involves several steps such as removal of silica, decrystalysation, hydrolyzation, and sugar/acid separation. The latter separation was performed using a strong acid resin to retain the sugars. Acid was used to regenerate the resin and obtain a 2% sugar solution.
U.S. Pat. No. 5,407,580 describes a method to separate acid from a non-ionic component such as sugar using ion exclusion.
U.S. Pat. No. 5,968,362 describes a method for separating acid and sugars from a biomass acid hydrolysis step. The process involves an anion-exchange resin or an ion-exclusion chromatographic material to retain the acid from the hydrolysate. The sugars produced are contaminated with acid and metals. The author proposes a treatment with lime to neutralise the solution and precipitate the metals.
U.S. Pat. No. 5,403,604 deals with sugar separation from juices using a set of membrane units including ultrafiltration, nanofiltration and reverse osmosis. The sugars are retained by the NF membrane while acids such as citric acid pass through. The total acid concentration in the feed stream is about 0.79 wt % while the total sugar varies from 4.3 to 14.3%.
U.S. Pat. No. 7,338,561 describes a process for purifying an aqueous solution containing sugars, multivalent cations, monovalent metal cations, monovalent anions and multivalent inorganic anions and/or organic acid anions. The process employs a strong anionic resin, a strong cationic resin, a nanofiltration device, a cristallisation unit, a reverse osmosis unit, and up to two chromatography columns. This approach was applied to a permeate from an ultrafiltration unit treating whey. The use of all of these units to perform the desired separation is complicated and does not seem to be economically attractive.
U.S. Pat. No. 7,077,953 deals with acid recovery from a hydrolysate solution obtained after exposing wood chips to an acidic solution. In this case, the sugars and the acid were contaminated with several other compounds such as lignin, metals, and suspended solids. A chromatographic unit is used to separate most of the sugars from the hydrolysis process. Water is employed to elute the sugars which are sent to a processing unit such as a fermentation/distillation unit. The chromatographic unit yields a dilute sugar stream which upon fermentation yields a diluted product that requires more energy to concentrate. The acid-rich stream from the chromatographic system is processed using a nanofiltration unit to remove the remaining sugars. The author also suggests a second nanofiltration unit ahead of the chromatographic unit to concentrate the sugars. However, in this case, monovalent metals and other ions such as chloride and potassium may accumulate in the acidic stream and cause fouling or corrosion of the metal surfaces during evaporation. In addition, in such a system, lignin is expected to accumulate in the concentrate or the sugar stream (permeate) causing its contamination. Metals or lignin present in the sugars may inhibit the fermentation of sugars to other valuable products such as ethanol. The author did not attempt to further fractionate the sugars.
U.S. Pat. No. 5,869,297 employs nanofiltration using polyimide nanofilters for the separation of dextrose. The feed solution contained higher saccharides such as disaccharides and trisaccharides.
U.S. Pat. No. 7,008,485 describes the use of nanofiltration to separate several small molar mass compounds from each other. The approach includes the separation of pentose sugars from hexose sugars, the separation of maltose from maltotriose, and the recovery of xylose from spent liquor. Ahead of the nanofilter a one or more pre-treatment steps such as ion exchange, ultrafiltration, chromatography, concentration, pH adjustment, filtration, dilution and crystallization may be required. Any combination of these units may be needed downstream in the nanofiltration process for further separation.
No attempt was made in the prior art to separate acid from the sugars and sugar oligomers from sugar monomers. The spent acid from an NCC plant is much purer than a typical biomass hydrolysate stream since the starting feedstock is bleached pulp which contains practically no lignin and metals. In addition, ion exchange uses chemicals to regenerate the resin and yields dilute streams. Hence, such an approach may be less economically attractive compared to the use of membranes, especially in the case of NCC production where the spent acid is purer than a biomass hydrolysate stream. A set of two nanofiltration units, having membranes with different molecular weight cut-offs, is sufficient to fractionate the spent acid into sugar monomers, sugar oligomers, and acid. Typically oligomers have a degree of polymerisation of 2 or higher. Therefore, the separation of the oligomers from the monomers requires a more open membrane than the one used for sugar/acid separation. This approach is much more economically viable compared to the processes mentioned above where several separation units are required.